The easiest method for forming an epitaxial semiconductor layer is generally to form a single crystal substrate made of the same material as that of a semiconductor to be grown, and to cause a vapor-phase growth of a semiconductor crystal on the resultant substrate, with actual success in various materials. However, because the single crystal substrate is not only technically difficult to obtain but also high in cost, a semiconductor crystal different from the substrate should often be grown on the substrate. In such a case, known combinations of the substrates and the semiconductor crystals are, for instance, GaAs on a silicon substrate, a nitride semiconductor on a sapphire or silicon carbide substrate, II–VI group semiconductors on a GaAs substrate, etc.
However, when a semiconductor different from a substrate is grown on the substrate, dislocations are likely introduced in a high density into a grown epitaxial semiconductor layer because of the mismatch of various characteristics such as lattice, a thermal expansion coefficient, surface energy, etc. Because the dislocations in the semiconductors may constitute non-radiative recombination centers, scattering centers, etc. in semiconductor devices such as optical devices, electronic devices, etc., devices using high-dislocation-density semiconductors are extremely poor in characteristics and stability.
Because bulk crystal growth is also difficult in a nitride semiconductor typically including gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), etc., the technology for producing single crystal substrates with practically acceptable sizes at a low cost have not been established yet. Accordingly, methods for causing nitride semiconductor crystals to grow epitaxially on substrates such as sapphire, silicon carbide, etc. are generally used. Still, dislocations remain serious problems as described above.
Under such circumstances, JP 8-8217 B proposes a method for reducing a dislocation density in a nitride semiconductor layer by “a two-stage growing method” using a metal-organic vapor-phase epitaxy method (MOVPE method). In this method, after a hydrogen gas is blown at as high a temperature as 1000° C. or higher onto a substrate surface of sapphire, etc. to remove an oxide film from the surface (heat cleaning), (a) a low-temperature buffer layer made of GaN, AlN, etc. is grown at 500–600° C. on the substrate, (b) the temperature is elevated to about 1000° C. for a heat treatment, and (c) a nitride semiconductor layer made of GaN, etc. is grown at about 1000° C. In the step (a), the crystal-growing temperature is lower than the melting point of GaN, AlN, etc., resulting in the formation of a polycrystalline, low-temperature buffer layer. In the subsequent step (b), the low-temperature buffer layer is partially turned to a single crystal by elevating the temperature to about 1000° C. to form fine crystal particles. In the step (c), an epitaxial layer made of GaN, etc. is formed with these fine crystal particles as nuclei. According to this method, the dislocation density of GaN on a sapphire substrate can be reduced from the level of conventional methods (1010–1011 per 1 cm2) to about 109 per 1 cm2.
As a method for reducing a dislocation density to 109 per 1 cm2 or less in a GaN semiconductor layer formed on a sapphire substrate, the inventors proposed a method of carrying out the following three steps of:    (i) forming fine crystal particles made of a nitride semiconductor on a substrate;    (ii) forming a nitride semiconductor island structure having pluralities of facets inclined relative to a surface of the substrate with the fine crystal particles as nuclei having a thickness of 0.1 to 1 μm; and    (iii) causing the nitride semiconductor island structure to grow in a direction parallel with a surface of the substrate to merge pluralities of the nitride semiconductor island structures with each other, thereby forming a nitride semiconductor crystal layer having a flat surface,under different conditions (method of prior invention), by U.S. Ser. No. 10/396,831 (filed on Mar. 26, 2003).
This method succeeds in making the dislocation density as small as 5×107 per 1 cm2 at minimum. To reduce the dislocation density to a level of 107 per 1 cm2 in this method, however, it is necessary to accelerate the growth of the above nitride semiconductor crystal in an island manner by adding Si at as high a concentration as 5×1019 atoms/cm3 to the crystal in the step of growing the nitride semiconductor having an island structure, or to cover the substrate surface partially with an Si layer by exposing the substrate surface to a gas containing an Si compound at a high concentration, and then cause fine crystal particles of the nitride semiconductor to grow on the coated Si layer.
Because a gas containing an Si compound at a high concentration should be introduced into a crystal-growing apparatus in these Si-using methods, Si remains on an inner wall of the crystal-growing apparatus, causing the problem that layers growing in subsequent steps, which should not contain Si, are contaminated with Si. On the other hand, the minimum dislocation density of a nitride semiconductor obtained by methods using no Si is as high as 1×108 per 1 cm2, failing to be reduced to a sufficient level.